realpath: Additional test cases

* Passing NULL should result in EINVAL
* Passing an empty path should result in ENOENT
* Failure with a non-null buffer should leave a partial result.  As
  pointed out in a comment in the test case, this reveals a discrepancy
  between the documentation and reality.

Sponsored by:	Klara, Inc.
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D53024

lib/libc/tests/gen/realpath2_test.c

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2017 Jan Kokemüller
  * All rights reserved.
+ * Copyright (c) 2025 Klara, Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -25,6 +26,8 @@
  */
 
 #include <sys/param.h>
+#include <sys/stat.h>
+
 #include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
@@ -34,6 +37,31 @@
 
 #include <atf-c.h>
 
+ATF_TC(realpath_null);
+ATF_TC_HEAD(realpath_null, tc)
+{
+	atf_tc_set_md_var(tc, "descr", "Test null input");
+}
+ATF_TC_BODY(realpath_null, tc)
+{
+	ATF_REQUIRE_ERRNO(EINVAL, realpath(NULL, NULL) == NULL);
+}
+
+ATF_TC(realpath_empty);
+ATF_TC_HEAD(realpath_empty, tc)
+{
+	atf_tc_set_md_var(tc, "descr", "Test empty input");
+}
+ATF_TC_BODY(realpath_empty, tc)
+{
+	char resb[PATH_MAX] = "";
+
+	ATF_REQUIRE_EQ(0, mkdir("foo", 0755));
+	ATF_REQUIRE_EQ(0, chdir("foo"));
+	ATF_REQUIRE_ERRNO(ENOENT, realpath("", resb) == NULL);
+	ATF_REQUIRE_STREQ("", resb);
+}
+
 ATF_TC(realpath_buffer_overflow);
 ATF_TC_HEAD(realpath_buffer_overflow, tc)
 {
@@ -44,16 +72,11 @@ ATF_TC_HEAD(realpath_buffer_overflow, tc)
 
 ATF_TC_BODY(realpath_buffer_overflow, tc)
 {
-	char path[MAXPATHLEN] = { 0 };
-	char resb[MAXPATHLEN] = { 0 };
-	size_t i;
+	char path[PATH_MAX] = "";
+	char resb[PATH_MAX] = "";
 
-	path[0] = 'a';
+	memset(path, 'a', sizeof(path) - 1);
 	path[1] = '/';
-	for (i = 2; i < sizeof(path) - 1; ++i) {
-		path[i] = 'a';
-	}
-
 	ATF_REQUIRE(realpath(path, resb) == NULL);
 }
 
@@ -66,9 +89,9 @@ ATF_TC_HEAD(realpath_empty_symlink, tc)
 
 ATF_TC_BODY(realpath_empty_symlink, tc)
 {
-	char path[MAXPATHLEN] = { 0 };
-	char slnk[MAXPATHLEN] = { 0 };
-	char resb[MAXPATHLEN] = { 0 };
+	char path[PATH_MAX] = "";
+	char slnk[PATH_MAX] = "";
+	char resb[PATH_MAX] = "";
 	int fd;
 
 	(void)strlcat(slnk, "empty_symlink", sizeof(slnk));
@@ -89,11 +112,77 @@ ATF_TC_BODY(realpath_empty_symlink, tc)
 	ATF_REQUIRE(unlink(slnk) == 0);
 }
 
+ATF_TC(realpath_partial);
+ATF_TC_HEAD(realpath_partial, tc)
+{
+	atf_tc_set_md_var(tc, "descr",
+	    "Test that failure leaves a partial result");
+	atf_tc_set_md_var(tc, "require.user", "unprivileged");
+}
+
+ATF_TC_BODY(realpath_partial, tc)
+{
+	char resb[PATH_MAX] = "";
+	size_t len;
+
+	/* scenario 1: missing directory */
+	ATF_REQUIRE_EQ(0, mkdir("foo", 0755));
+	ATF_REQUIRE_ERRNO(ENOENT, realpath("foo/bar/baz", resb) == NULL);
+	len = strnlen(resb, sizeof(resb));
+	ATF_REQUIRE(len > 8 && len < sizeof(resb));
+	ATF_REQUIRE_STREQ("/foo/bar", resb + len - 8);
+
+	/* scenario 2: dead link 1 */
+	ATF_REQUIRE_EQ(0, symlink("nix", "foo/bar"));
+	ATF_REQUIRE_ERRNO(ENOENT, realpath("foo/bar/baz", resb) == NULL);
+	len = strnlen(resb, sizeof(resb));
+	ATF_REQUIRE(len > 8 && len < sizeof(resb));
+	ATF_REQUIRE_STREQ("/foo/nix", resb + len - 8);
+
+	/* scenario 3: missing file */
+	ATF_REQUIRE_EQ(0, unlink("foo/bar"));
+	ATF_REQUIRE_EQ(0, mkdir("foo/bar", 0755));
+	ATF_REQUIRE_ERRNO(ENOENT, realpath("foo/bar/baz", resb) == NULL);
+	len = strnlen(resb, sizeof(resb));
+	ATF_REQUIRE(len > 12 && len < sizeof(resb));
+	ATF_REQUIRE_STREQ("/foo/bar/baz", resb + len - 12);
+
+	/* scenario 4: dead link 2 */
+	ATF_REQUIRE_EQ(0, symlink("nix", "foo/bar/baz"));
+	ATF_REQUIRE_ERRNO(ENOENT, realpath("foo/bar/baz", resb) == NULL);
+	len = strnlen(resb, sizeof(resb));
+	ATF_REQUIRE(len > 12 && len < sizeof(resb));
+	ATF_REQUIRE_STREQ("/foo/bar/nix", resb + len - 12);
+
+	/* scenario 5: unreadable directory */
+	ATF_REQUIRE_EQ(0, chmod("foo", 000));
+	ATF_REQUIRE_ERRNO(EACCES, realpath("foo/bar/baz", resb) == NULL);
+	len = strnlen(resb, sizeof(resb));
+	ATF_REQUIRE(len > 8 && len < sizeof(resb));
+	/*
+	 * This is arguably wrong.  The problem is not with bar, but with
+	 * foo.  However, since foo exists and is a directory and the only
+	 * reliable way to detect whether a directory is readable is to
+	 * attempt to read it, we do not detect the problem until we try
+	 * to access bar.
+	 */
+	ATF_REQUIRE_STREQ("/foo/bar", resb + len - 8);
+
+	/* scenario 6: not a directory */
+	ATF_REQUIRE_EQ(0, close(creat("bar", 0644)));
+	ATF_REQUIRE_ERRNO(ENOTDIR, realpath("bar/baz", resb) == NULL);
+	len = strnlen(resb, sizeof(resb));
+	ATF_REQUIRE(len > 4 && len < sizeof(resb));
+	ATF_REQUIRE_STREQ("/bar", resb + len - 4);
+}
+
 ATF_TP_ADD_TCS(tp)
 {
-
+	ATF_TP_ADD_TC(tp, realpath_null);
+	ATF_TP_ADD_TC(tp, realpath_empty);
 	ATF_TP_ADD_TC(tp, realpath_buffer_overflow);
 	ATF_TP_ADD_TC(tp, realpath_empty_symlink);
+	ATF_TP_ADD_TC(tp, realpath_partial);
 
 	return atf_no_error();
 }